The nature and properties of singlet molecular oxygen and its role in photodynamic action will continue to be a major focus of this research group in the next five years. The experimental study will be broadly divided into two categories: one is concerned with the basic physico- chemical aspects of how oxygen interacts with electronically excited systems; the other centered on biological aspects, in particular, photodynamic mechanisms. Two specific aims constitute the first category. Briefly, these are focused on demonstrating and characterizing the dynamic properties of exciplex states formed by photosensitizer-oxygen interactions. This will involve both diffusive interactions, and excitation of molecule-oxygen compound states. Three other aims concern research on cellular systems and on photoprocesses therein. These studies will include investigations of the reactivity of molecular triplet states in cultured cells, an attempt to develop a relationship between singlet oxygen production and biological effect, and to continue our efforts to understand the early effects of photodynamic irradiation on cellular homeostasis. Most of the physico-chemical work will involve laser flash photolytic investigations. Several instrumental developments are planned for the cellular work, including an investigation into the feasibility of developing an interferometer-based luminescence device for enhanced sensitivity and the employment of light-integrating enclosures to enhance absorption and emission measurements.